How to mock a python class dependency using pytest-mock?

Question

I am struggling to understand how to stub a class / mock all the methods from a class dependency in Python & pytest. The listing below shows the class that I am testing. It has two internal dependencies: OWMProxy and PyOwmDeserializer.

Class Under Test

class OWM:
    def __init__(self, api_key: str, units: WeatherUnits) -> None:
        self._api_key = api_key
        self._units = units    
        self._pyorm = OWMProxy.from_api_key(api_key)
        self._deserializer = PyOwmDeserializer()

    def at(self, city: str, iso_datetime: str) -> ForecastModel:
        weather = self._pyorm.for_time(city, iso_datetime)

        return self._deserializer.deserialize(weather, self._units)

    def day(self, city: str, day: str) -> ForecastModel:
        weather = self._pyorm.for_day(city, day)

        return self._deserializer.deserialize(weather, self._units)

    def now(self, city: str) -> ForecastModel:
        weather = self._pyorm.now(city) 

        return self._deserializer.deserialize(weather, self._units)

My question is, is it possible to mock an entire class dependency when unit test with PyTest?

Currently, the unit test that I have uses mocker to mock each class method, including init method.

I could use a dependency injection approach, i.e. create an interface for the internal deserializer and proxy interface and add these interfaces to the constructor of the class under test.

Alternatively I could test using unittest.mock module, as suggested here. Is there an equivalent functionality in pytest-mock???

Unit Test So Far...

@pytest.mark.skip(reason="not implemented")
  def test_owm_initialises_deserializer(
      default_weather_units: WeatherUnits, mocker: MockFixture
  ) -> None:
      api_key = "test_api_key"
  
      proxy = OWMProxy(py_OWM(api_key))
  
      patch_proxy = mocker.patch(
          "wayhome_weather_api.openweathermap.client.OWMProxy.from_api_key",
          return_value=proxy,
      )   
  
      patch_val = mocker.patch(
          "wayhome_weather_api.openweathermap.deserializers.PyOwmDeserializer",
          "__init__",
          return_value=None,
      )   
  
      owm = OWM(api_key, default_weather_units)
  
      assert owm is not None

Answer 1

You can just mock the whole class and control the return values and/or side effects of its methods as how its done in the docs.

>>> def some_function():
...     instance = module.Foo()
...     return instance.method()
...
>>> with patch('module.Foo') as mock:
...     instance = mock.return_value
...     instance.method.return_value = 'the result'
...     result = some_function()
...     assert result == 'the result'

Assuming the class under test is located in src.py.

test_owm.py

import pytest
from pytest_mock.plugin import MockerFixture

from src import OWM, WeatherUnits


@pytest.fixture
def default_weather_units():
    return 40


def test_owm_mock(
      default_weather_units: WeatherUnits, mocker: MockerFixture
) -> None:
    api_key = "test_api_key"

    #  Note that you have to mock the version of the class that is defined/imported in the target source code to run. So here, if the OWM class is located in src.py, then mock its definition/import of src.OWMProxy and src.PyOwmDeserializer
    patch_proxy = mocker.patch("src.OWMProxy.from_api_key")
    patch_val = mocker.patch("src.PyOwmDeserializer")

    owm = OWM(api_key, default_weather_units)

    assert owm is not None

    # Default patch
    print("Default patch:", owm.day("Manila", "Today"))

    # Customizing the return value
    patch_proxy.return_value.for_day.return_value = "Sunny"
    patch_val.return_value.deserialize.return_value = "Really Sunny"
    print("Custom return value:", owm.day("Manila", "Today"))
    patch_proxy.return_value.for_day.assert_called_with("Manila", "Today")
    patch_val.return_value.deserialize.assert_called_with("Sunny", default_weather_units)

    # Customizing the side effect
    patch_proxy.return_value.for_day.side_effect = lambda city, day: f"{day} in hot {city}"
    patch_val.return_value.deserialize.side_effect = lambda weather, units: f"{weather} is {units} deg celsius"
    print("Custom side effect:", owm.day("Manila", "Today"))
    patch_proxy.return_value.for_day.assert_called_with("Manila", "Today")
    patch_val.return_value.deserialize.assert_called_with("Today in hot Manila", default_weather_units)


def test_owm_stub(
      default_weather_units: WeatherUnits, mocker: MockerFixture
) -> None:
    api_key = "test_api_key"

    class OWMProxyStub:
        @staticmethod
        def from_api_key(api_key):
            return OWMProxyStub()

        def for_day(self, city, day):
            return f"{day} in hot {city}"

    class PyOwmDeserializerStub:
        def deserialize(self, weather, units):
            return f"{weather} is {units} deg celsius"


    patch_proxy = mocker.patch("src.OWMProxy", OWMProxyStub)
    patch_val = mocker.patch("src.PyOwmDeserializer", PyOwmDeserializerStub)

    owm = OWM(api_key, default_weather_units)

    assert owm is not None

    # Default patch
    print("Default patch:", owm.day("Manila", "Today"))
    # If you want to assert the calls made as did in the first test above, you can use the mocker.spy() functionality

Output

$ pytest -q -rP
================================================================================================= PASSES ==================================================================================================
______________________________________________________________________________________________ test_owm_mock ______________________________________________________________________________________________
------------------------------------------------------------------------------------------ Captured stdout call -------------------------------------------------------------------------------------------
Default patch: <MagicMock name='PyOwmDeserializer().deserialize()' id='139838844832256'>
Custom return value: Really Sunny
Custom side effect: Today in hot Manila is 40 deg celsius
______________________________________________________________________________________________ test_owm_stub ______________________________________________________________________________________________
------------------------------------------------------------------------------------------ Captured stdout call -------------------------------------------------------------------------------------------
Default patch: Today in hot Manila is 40 deg celsius
2 passed in 0.06s

As you can see, we are able to control the return values of the methods of the mocked dependencies.